The present invention is directed to an infrared motion alarm comprising an infrared detector and an optical arrangement preceding the detector in the radiation direction for focussing the infrared radiation onto the detector. The optics are formed by a lens arrangement adjacent the detector and a mirror arrangement which will reflect the radiation through the lens into the detector.
An infrared motion alarm, which has an infrared detector and an optical arrangement for focussing infrared radiation into the detector, will serve for an excursion or burgular alarm. However, there is a demand for protection against crawling under the sensed beam of radiation or in other words, to determine movement below a detection zone for the motion alarm if possible so that the detection zone will proceed from wall to wall with an aperture angle of 180.degree..
Infrared motion alarms are known which are composed of a combination of a mirror and lens arrangement. Examples are U.S. Pat. Nos. 3,766,539 and 3,631,434 and the disclosures of these two patents are incorporated by reference thereto. As a consequence of the simple design of these known infrared motion alarms and the acquisition of the radiation in only one plane; however, faultless protection against a crawling individual under the path of detected rays is therefore not guaranteed.